Cold-retreading a tyre comprises removing the worn tread from the tyre, and applying a new, cured tread to the tyre carcass. Applying the new, cured tread to the carcass comprises winding a green-rubber cushion and a so-called PCT (Pre-Cured Tread) strip around the carcass; and the carcass, wound inside the cushion and the pre-cured tread strip, is then inserted inside an autoclave and cured further to grip the tread firmly to the carcass by means of the binding action of the cushion.
One example of a pre-cured tread strip, ready for application to a tyre carcass, is described in U.S. Pat. No. 5,277,727A1. One example of a cold tyre retreading station employing the above method is described in U.S. Pat. No. 6,368,439B1.
Constructing the pre-cured tread strip comprises extruding a green-rubber compound into a continuous green tread strip, which is then cut crosswise into a succession of green tread strips of given length. Each green tread strip is inserted inside a flat curing mold, in which it is cured; and, at the end of the curing process, the cured tread strip must be extracted from the curing mold by detaching the cured tread strip from the parts of the curing mold defining a negative of the tread pattern. The flat curing mold is normally equipped with a gripper, which is initially integrated in the curing mold, engages one end of the cured tread strip, and is lifted up and drawn along by an actuating device to gradually extract the cured tread strip from the curing mold.
Two examples of extracting a cured tread strip from a flat curing mold are described in Patent Applications US2004197432A1 and US2011148001A1.
As it is extracted from the flat curing mold, the cured tread strip bends roughly 180° with a small bend radius in the extraction area, thus subjecting the rubber to severe mechanical stress concentrated in the extraction area, and which may result in the formation of cracks (i.e. deep, narrow fissures) normally originating at the grooves in the tread pattern. In which case, the cured tread strip must obviously be scrapped.
One proposed solution, to reduce mechanical stress concentrated in the extraction area, is to employ a guide roller located over the curing mold, and about which the extracted part of the cured tread strip winds. The guide roller is mounted idly to rotate freely about its central axis, and also to move freely in a direction parallel to the curing mold. The guide roller forces the cured tread strip to bend 180° with a bend radius no smaller than the radius of the guide roller itself. So, a guide roller with a large enough radius prevents the cured tread strip from bending 180° with too small a bend radius, and so prevents severe mechanical stress, capable of forming cracks, from concentrating in the cured tread strip in the extraction area. Relatively large-radius guide rollers, however, have been found to impede extraction of the cured tread strip from the flat curing mold, by forming an overhead obstacle, with which the cured tread strip underneath collides in the extraction area.